1. Field of the Invention
The present invention relates to a laser beam machining apparatus for performing laser beam machining along streets formed in a surface of a wafer held on a chuck table.
2. Description of the Related Art
In the semiconductor device manufacturing process, a plurality of regions are demarcated by planned dividing lines called streets arranged in a grid pattern in a surface of a substantially circular disk-shaped semiconductor wafer, and circuits such as ICs and LSIs are formed in the thus demarcated regions. Then, the regions with the circuits formed therein are divided by cutting the semiconductor wafer along the streets, to manufacture the individual semiconductor chips. Similarly, an optical device wafer in which a gallium nitride compound semiconductor and the like are laminated on a surface of a sapphire substrate is also cut along streets, to be divided into individual optical devices such as light emitting diodes and laser diodes, which are widely utilized for electric apparatuses.
In recent years, as a method for dividing a plate-shaped work such as a semiconductor wafer, a laser beam machining method has been attempted in which a pulsed laser beam capable of being transmitted through the work is used, and irradiation with the pulsed laser beam is conducted by adjusting the converging point in the inside of the regions to be divided. In a dividing method based on the use of this laser beam machining method, a work is irradiated from one side thereof with a pulsed laser beam having such a wavelength (for example, 1064 nm) as to be transmitted through the work while adjusting the converging point to the inside of the work, so as to continuously form a denatured layer in the inside of the work along the streets, and an external force is exerted along the streets where strength is lowered due to the formation of the denatured layer, thereby dividing the work (refer to, for example, Japanese Patent No. 3408805).
Meanwhile, an arcuate chamfer part is formed at the outer periphery of the wafer, and, at the time when the outer peripheral part provided with the chamfer part is irradiated with a laser beam, the converging point of the laser beam may coincide with the surface of the chamfer part. When the converging point of the laser beam thus coincides with the surface of the chamfer part, ablation machining may occur to generate debris, which would be deposited on the devices, thereby lowering the device quality. In addition, once the ablation machining occurs, the ablation machining tends to occur successively even if the converging point of the laser beam is positioned to the inside of the wafer.